Currently it is common practice, for making printed circuit boards, to purchase insulated panels or boards, both of whose opposite main surfaces are copper clad, and then to pattern the copper cladding to achieve the desired circuitry. These copper-clad boards are prepared by pressing a copper foil on the board after first providing a thin zinc-copper layer there between to promote adhesion between the copper and the board. The copper cladding typically is about 0.7 mil thick, which is thicker than optimum, but because of the process used for cladding, it is difficult to provide thinner copper layers reliably and inexpensively. For use in printed circuit boards layers of this thickness require unduly long times and large amounts of chemicals for patterning and so increase the cost. Also such thicknesses can result in high steps at pattern edges which can give rise to problems. There accordingly is a need for copper-coated boards with thinner copper coatings for use in making printed circuit boards and the present invention is directed to this end.
Printed circuit boards with relatively thin layers of electroless copper, which are held to the boards by adhesive, are commercially available but are not as reliable as is required in some applications.
One proposed solution has been to prepare the boards by electroless copper plating. However, it has proven difficult, even with the use of adhesives, to provide coatings of the desired thinness which are continuous and uniform and have the required high resistance to peeling under operating conditions.